Neutron Leakage Benchmarks for Water Moderators

Fission reaction rates for four nuclides were measured in the leakage spectrum outside spherical water moderators of various radii surrounding a 252Cf neutron source. Using the MCNP transport code, matching calculations were made with highly detailed modeling of the measurement apparatus. The calculations predicted significantly higher leakage of neutrons in the epicadmium energy range than was found in the measurements. A discrepancy of the same sign but weaker magnitude was found for thermal neutrons. These discrepancies may be relevant to problems with criticality calculations in special cases.